During the 1880's, an infrared detector called the bolometer was developed. The bolometer operates on the principle that the electrical resistance of the bolometer material changes with respect to the bolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics can be exploited to measure incident infrared radiation on the bolometer by sensing the resulting change in its resistance. When used as an infrared detector, the bolometer is generally thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the bolometer material.
Modern microbolometer structures were developed by the Honeywell Corporation. For a recent summary of references see U.S. Pat. No. 5,420,419. Microbolometer arrays are typically fabricated on monolithic silicon substrates or integrated circuits by constructing two-dimensional arrays of closely spaced air bridge structures coated with a temperature sensitive resistive material, such as vanadium oxide, that absorbs infrared radiation. The air bridge structure provides good thermal isolation between the microbolometer detector and the silicon substrate. A typical microbolometer structure measures approximately 50 microns by 50 microns.
Microbolometer arrays can be used to sense a focal plane of incident radiation (typically infrared) by absorbing the radiation and producing a corresponding change in the temperatures and therefore resistances of each microbolometer in the array. With each microbolometer functioning as a pixel, a two-dimensional image or picture representation of the incident infrared radiation can be generated by translating the changes in resistance of each microbolometer into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a computer. The circuitry used to perform this translation is commonly known as the Read Out Integrated Circuit (ROIC), and is fabricated as an integrated circuit in the silicon substrate. The microbolometer array is then fabricated on top of the ROIC. The combination of the ROIC and microbolometer array is commonly known as a microbolometer infrared Focal Plane Array (FPA). Microbolometer FPAs containing as many as 320.times.240 detectors have been demonstrated.
Individual microbolometer detectors will have non-uniform responses to uniform incident infrared radiation, even when the bolometers are manufactured as part of a microbolometer FPA. This is due to small variations in the detectors' electrical and thermal properties as a result of the manufacturing process. These non-uniformities in the microbolometer response characteristics must be corrected to produce an electrical signal with adequate signal-to-noise ratio for image display or processing.
Under the conditions where uniform electrical bias and incident infrared radiation are applied to an array of microbolometer detectors, differences in detector response will occur. This is commonly referred to as spatial non-uniformity, and is due to the variations in a number of critical performance characteristics of the microbolometer detectors. This is a natural result of the microbolometer fabrication process. The characteristics contributing to spatial non-uniformity include the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors.
The magnitude of the response non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation. The resulting ROIC output signal is difficult to process, as it requires system interface electronics having a very high dynamic range. In order to achieve an output signal dominated by the level of incident infrared radiation, processing to correct for detector non-uniformity is required.
Previous methods for implementing an ROIC for microbolometer arrays have used an architecture wherein the resistance of each microbolometer is sensed by applying a uniform voltage and current and a resistive load to the microbolometer element. The current resulting from the applied voltage is integrated over time by an amplifier to produce an output voltage level proportional to the value of the integrated current. The output voltage is then multiplexed to the signal acquisition system.
Gain and offset corrections are applied to the output signal to correct for the errors which arise from the microbolometer property non-uniformities. This process is commonly referred to as two-point correction. In this technique two correction coefficients are applied to the sampled signal of each element. The gain correction is implemented by multiplying the output voltage by a unique gain coefficient. The offset correction is implemented by an adding a unique offset coefficient to the output voltage. Both analog and digital techniques have been utilized to perform two-point non-uniformity correction.
The current state-of-the-art in microbolometer array ROICs suffers from two principal problems. The first problem is that the large magnitude of the microbolometer introduced non-uniformities in the ROIC output signal requires a large instantaneous dynamic range in the sensor interface electronics, increasing the cost and complexity for the system. Current advanced ROIC architectures incorporate part of the correction on the ROIC to minimize the instantaneous dynamic range requirements at the acquisition systems interface.
The second problem is that the application of a fixed coefficient two-point gain and offset correction method to minimize array non-uniformity works well only for a very small range of substrate temperatures, on the order of 0.005 to 0.025 degrees Kelvin. In order to maintain the substrate temperature within this range, a thermo-electric cooler, temperature sensor, and temperature control electronics are required, again adding to system cost and complexity.
Microbolometer Operation
FIGS. 1A, 1B and 1C illustrate three possible configurations for microbolometer detectors. Incident infrared radiation 1 is projected onto each of the microbolometer detectors. The microbolometer detector 2, shown in FIG. 1A, is thermally-shorted to the substrate material. This is a common form of bolometer and is representative of most single-detector bolometers and thermistors. Microbolometers can be manufactured to provide a high thermal conductivity to the substrate, or this property can be introduced through post-processing whereby a thermally conductive material is selectively applied to these microbolometer detectors. Although this detector is thermally connected to the substrate, the resistance properties and temperature coefficient of resistance (TCR) of these detectors are equivalent to the thermally isolated microbolometer detector 3, shown in FIG. 1B. These detectors have a high TCR (1% to 5%/.degree. C.) which is designated by the arrow in the resistor symbol.
The thermally isolated microbolometer 3 is most commonly used to sense infrared radiation. Microbolometer 3 is thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the microbolometer material. In FIG. 1B, this isolation is designated by the dashed square box around the detector.
The final configuration, shown in FIG. 1C, is the shielded microbolometer 4. The shielded microbolometer 4 is identical to the isolated microbolometer 3 with the exception that incident infrared radiation 1 is shielded from the microbolometer. The radiation shield is designated by a solid line 5 in FIG. 1C.
The principles of operation for the microbolometers shown in FIGS. 1A-1C are as follows. The temperature of the non-isolated microbolometer 2 is dominated by the high thermal conductivity to the substrate. Therefore incident infrared radiation and electrical power dissipated in the microbolometer have little effect on the temperature of the microbolometer. Microbolometer 2 has the same high TCR as the thermally isolated microbolometer 3 and therefore provides a high sensitivity reference to the substrate temperature.
The thermally isolated microbolometer 3 changes temperature in response to the incident radiation level, changes in the substrate temperature, and the electrical power dissipated in the detector during the measurement of the microbolometer resistance. The heating due to resistive measurement is referred to as self-heating. As fabricated, the thermally isolated microbolometer is not perfectly insulated from the substrate. Therefore the temperature of the thermally isolated microbolometer does track the substrate temperature to some extent, although the rate of temperature change due to this mechanism is much slower than those due to incident radiation or self-heating.
The shielded isolated microbolometer 4 does not change temperature in response to the incident radiation level, but does change temperature as a result of self heating and temperature changes in the substrate.
FIGS. 2-4 illustrate three commonly used techniques for measuring the resistance of the microbolometer detector. FIG. 2 shows the applied voltage method of sensing the detector resistance. An applied voltage is used to generate a current in the circuit designated Iout. By measuring the current Iout the resistance of the microbolometer detector can be determined. The relationship between the applied voltage and measured current is defined by Ohms Law. ##EQU1## Where Iout is the measured current, Vapplied is the applied voltage and Rbolometer is the microbolometer detector resistance.
A second method for measuring the microbolometer resistance is shown in FIG. 3. Here a constant current is applied to the microbolometer detector 3, and the voltage that develops across the microbolometer as a result is measured. Again Ohms Law defines the relationship between the applied current and the measured voltage. EQU Vout=Iapplied*Rbolometer
A third method for measuring the microbolometer resistance is shown in FIG. 4. This circuit includes a resistive load 6. A voltage is applied across the series combination of the microbolometer 3 and the load 6. The microbolometer resistance can be determined by measuring the voltage across the microbolometer. The following expression describes the microbolometer resistance as a function of applied voltage, load resistance, and the measured voltage across the microbolometer. ##EQU2## where Rload is the value of load resistor 6.
These circuit implementations can be used to measure infrared radiation incident on the microbolometer by sensing the change in microbolometer temperature due to the optical (infrared) energy absorbed by the detector. The temperature rise in the microbolometer detector due to self-heating generally is significantly larger than the temperature rise resulting from the incident infrared radiation. The relatively small contribution of incident radiation to the change in microbolometer resistance is difficult to detect. For this reason, it is desirable to incorporate more complex circuits using in-circuit reference schemes in order to minimize the contribution of self-heating to the output signal. In the case of the resistive load circuit approach (FIG. 4), the load resistor 6 may be designed to have a low temperature coefficient of resistance, or it may be thermally-shorted to the substrate, or shielded from incident infrared radiation.
Circuit bridge concepts have been designed to minimize the errors in resistance measurement due to self-heating. FIG. 5 illustrates a microbolometer bridge concept used to isolate and measure only the level of incident infrared radiation. Here a microbolometer that is thermally isolated and shielded from incident radiation is shown in a bridge configuration with three conventional low TCR resistors, 6a, 6b and 6c. The output voltage in FIG. 5 will remain relatively constant in spite of a change in ambient temperature, because the increase in resistance of the microbolometer detector and the shielded microbolometer will increase by approximately the same amount and keep the voltage drops across the elements of the bridge circuit approximately unchanged.
Microbolometer Focal Plane Arrays
In systems where a single detector is employed, two conductive leads can be attached to the microbolometer material providing a means of conducting current through the microbolometer to sense its resistance. FIG. 6 illustrates the electrical connection to the microbolometer detector. In this case, a thermally isolated microbolometer 3 is shown in the presence of incident infrared radiation 1 with two leads connecting to microbolometer terminals R+ and R-. FIG. 7 shows the physical implementation of a microbolometer of the kind developed by Honeywell. The R+ and R- electrical connections to the microbolometer are created at the ends of the legs 9 where the microbolometer comes in contact with the substrate 8.
In cases where it is desired to sense the resistance or temperature of an array of microbolometer detectors it becomes physically impractical to provide individual wire lead connections for each detector. FIGS. 8 and 9 illustrate the method of interconnecting to a microbolometer detector array. Shown in FIGS. 8 and 9 is a three-by-three detector array requiring nine positive and negative interconnects. Interconnects for the individual microbolometer detectors 3 in the array are created as part of the fabrication process, and contact the circuitry in the silicon substrate 8.
Large two-dimensional arrays of microbolometers can utilize a Read Out Integrated Circuit (ROIC) to provide the required bolometer interface. The ROIC incorporates circuitry that is placed in spatial proximity to the detectors to perform the functions of the detector interface and multiplexing. The circuitry associated with a particular microbolometer detector is often located in the silicon substrate directly beneath the detector and is referred to as the unit cell.
By time-multiplexing signals of the microbolometer detectors the number of required electrical interconnect leads can be greatly reduced. FIG. 10 shows schematically a one-dimensional multiplexing scheme for a microbolometer array. Three microbolometer detectors 3 are multiplexed to a single column amplifier 15. A row enable line 16 is used to control an address switch 10 in a unit cell 14. This allows selective connection of the unit cell bolometers to the column amplifier 12. The current through each microbolometer detector will be sequentially sampled for integration by the amplifier. The order of sequencing and the time period of each sample is determined by the sequencing and duration of each row enable signal's active period. In this embodiment a uniform bias 11 is applied to the microbolometer array at the time each detector is addressed. A conventional inverting amplifier 12, and its feedback element 13 is shown in the column amplifier block 15. In an actual ROIC the address switches 10 shown in FIG. 10 would be implemented as MOS or bipolar transistors, and are shown as switches to simplify the illustration.
Multiplexing can be expanded to a second dimension by arraying the one-dimensional configuration shown in FIG. 10. The resulting two-dimensional three-by-three configuration is shown in FIG. 11. The two-dimensional array is implemented by adding column interconnects and incorporating column multiplexing switches 18. Column amplifiers 15 shown in FIG. 10 have been modified to incorporate a sample and hold stage 12A to allow time-simultaneous sampling of the signal in the column dimension. Outputs from sample and hold stages 12A are selected by the column enable signal 19 that controls the column switch 18. An output line 17 common to all columns is used to bus the output signals from the column amplifiers 15 to the ROIC output.
To simplify the multiplexing process and system interface, the ROIC contains digital logic circuitry to generate the signals required to control the row and column address switches. FIGS. 12 and 13 show an implementation of logic circuitry capable of generating the addressing signals for the row and column address switches. In each case a chain of D-flip flops 21 is used to shift an addressing signal through the row and column enables. The multiplexing process is performed by enabling a row and then sequentially enabling the column selects.
The addressing synchronization signals RowSync and ColumnSync (FIGS. 12 and 13 respectively) are inputs to the ROIC or are generated by on-ROIC logic. These signals are driven to the first D-flop's 21 "D" terminal and inverted by inverter 20 to the "D-bar" terminal. Row and column clocks are used to shift the sync pulses down the shift registers. AND gates 22 are used to decode a unique addressing state for each of the Row Enable and Column Enable outputs.
FIG. 14 illustrates the placement of the components to provide a ROIC for an 8.times.8 array of microbolometer detectors. The array of unit cells, column amplifiers, a column multiplexer 25 and a row 26 multiplexer are integrated on to a single ROIC silicon die. The microbolometer array is constructed on top of the unit cell array. In addition to the circuits previously described, bias generation and timing control circuitry 24 and an output amplifier 27 are included. The ROIC provides critical interfaces for both the microbolometer detector array and the external system.
Microbolometer Non-Uniformity Correction
When uniform electrical bias and incident infrared radiation are applied to an array of microbolometer detectors, differences in detector response will occur. As noted above, this is commonly referred to as spatial non-uniformity, and is due to the distribution in values of a number of critical performance characteristics of the microbolometer detectors, a natural result of the microbolometer fabrication process. The characteristics contributing to detector non-uniformity include the detectors' infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity.
The magnitude of the response non-uniformity can be substantially larger than the magnitude of the response due to the incident infrared radiation. The resulting ROIC output signal is difficult to process as it requires system interface electronics having a very high dynamic range. In order to achieve an output signal dominated by the response due to incident infrared radiation, processing to correct for detector non-uniformity is required.
FIG. 15 shows a conventional method to correct for microbolometer non-uniformities. A single detector signal path is shown for simplicity. Here a uniform bias 11 is applied to all of the microbolometer array detectors 3. Current from the microbolometer is integrated by the integration stage 28. Offset 29 and gain 30 correction functions are shown at the output of the integration stage. The offset and gain corrections are addition and multiplication functions, respectively. These functions together are commonly referred to as two-point correction. It is possible for the offset and gain corrections to be implemented as a part of the integration stage or after the integration stage, on or off of the ROIC, and either in analog or digital form. In addition, the offset correction can be performed either before or after the gain correction.
FIGS. 16A, 16B and 16C illustrate the traditional two-point correction technique. The graph in FIG. 16A shows the transfer function for two detectors having different responses to the same optical (infrared) stimulus. Qmin and Qmax are the maximum and minimum anticipated levels of infrared radiation, respectively. The graph in FIG. 16B shows the application of offset correction. Offset correction coefficients are acquired at Qmin for this operation. The graph in FIG. 16C shows the application of both offset and gain correction. Here gain coefficients are calculated for signal response between Qmin and Qmax. For arrays with linear signal transfer functions, this technique can provide a high degree of correction and produce an image that is pleasing to the eye. However, due to the high sensitivity of microbolometer arrays to substrate temperature, traditional two-point correction methods are successful for only a small range of substrate temperatures.
Values for the gain and offset correction terms, or coefficients, are specific to each microbolometer detector and are generated and stored during a calibration process. For a constant substrate temperature, two uniform infrared illumination levels (such as Qmin and Qmax) can be used to acquire the gain and offset correction coefficients. At a substrate temperature T.sub.1 and uniform infrared illumination level Qmin, the signal outputs of the detectors can be used to derive the offset coefficients. By then changing the uniform infrared illumination to a different level Qmax while maintaining the substrate temperature at T.sub.1, the gain coefficients can be calculated from the output signals.
The gain and offset correction coefficients generated in this way are normally stored in a correction coefficient memory. The output signal from the sensor is converted to digital form and is processed through gain and offset (multiply and addition) processes. Correction coefficient data are retrieved from the correction coefficient memory and applied to the output data in the multiply and addition processes.
The two-point correction process is further described in C. G. Bethea et al., "Long Wavelength Infrared 128.times.128 Al.sub.x Ga.sub.1-x As/GaAs Quantum Well Infrared Camera and Imaging System", IEEE Transactions On Electron Devices, Vol. 40, No. 11, November 1993, pp. 1957-1963, which is incorporated herein by reference in its entirety.
The conventional two-point correction process can provide spatial non-uniformity correction below the theoretical temporal noise of the microbolometer detectors. This correction technique is only effective, however, for a small range of microbolometer substrate temperature variations (on the order of 0.01 degree Kelvin).
FIGS. 17A-17C illustrate graphs showing the number of elements in a microbolometer array (vertical axis) having a given signal output (horizontal axis) in the presence of uniform input signals (bias and incident optical radiation). FIG. 17A shows the simulated uncorrected signal distribution for a microbolometer array. FIG. 17B shows the simulated resulting distribution after applying a two-point correction. FIG. 17C shows the simulated resulting signal distribution after the microbolometer substrate temperature has been changed.
It is thus apparent that microbolometer arrays have a large sensitivity to the ROIC substrate temperature. Changes in the substrate temperature introduce substantial errors to the non-uniformity correction results. FIG. 18 shows the two-point corrected signal distribution for an array of microbolometers as a function of substrate temperature. Temperature T.sub.1 was used as the substrate temperature for the calibration process. For a substrate temperature equal to T.sub.1, near ideal spatial non-uniformity correction is achieved. As the substrate temperature moves away from T.sub.1 the spatial non-uniformity rapidly increases.
FIG. 19 shows the non-uniformity vs. optical signal for a microbolometer ROIC array corrected using the traditional two-point method. The vertical axis is plotted as sigma/mean of the array output signal, which is a measure of the spatial non-uniformity of the ROIC array. Qmin and Qmax are the optical illumination levels used to generate the two-point gain and offset correction coefficients. At optical illumination levels above Qmax or below Qmin the spatial non-uniformity for the sensor degrades rapidly. Also note that the spatial non-uniformity degrades between Qmin and Qmax, but to a limited extent due to nonlinearity.
For a microbolometer sensor array that has been corrected using the traditional two-point method with substrate temperature at Tnominal (the midpoint between the maximum and minimum expected temperatures), the spatial non-uniformity degrades rapidly when the substrate temperature is changed from Tnominal. This effect is shown in FIG. 20. Two points are shown where the spatial non-uniformity is at a minimum. These are at Qmin and Qmax where the substrate is at Tnominal. As the substrate temperature changes, the spatial non-uniformity and sigma/mean degrades rapidly.
Systems utilizing past microbolometer infrared detector technologies have required cooling systems, vacuum packaging systems and complex processing electronics to maintain the substrate temperature within a very tight tolerance (e.g., 0.01 degrees Kelvin). The added cost of these systems has impeded the development of a high-volume, low-cost commercial market for infrared imaging systems. A microbolometer infrared detector array which did not require high tolerance cooling would have the potential to become the first infrared technology to allow penetration into high-volume, low-cost commercial markets.